It is now known that a variety of skin diseases and conditions in which the skin has undergone some form of damage can be exacerbated by, or traced directly or indirectly to, solar radiation exposure, as is damage to hair. Such is the case, for example, in ultraviolet-induced skinburn and accelerated skin aging caused by sunlight. Continued sun exposure can lead to pre-cancers and cancers, including melanoma. Hence, there is currently considerable public health and cosmetic interest in the promotion of sunscreen and sunblock products for use on exposed skin surfaces.
Conventional sunscreens are typically prepared by suspending or dissolving an organic compound that absorbs and/or attenuates harmful ultraviolet (UV) radiation, such as, for example, oxybenzone(2-hydroxy-4-methoxybenzophenone), doxybenzone(2,2′)-dihydroxy-4-methoxybenzophenone), aminobenzoic acid, cinoxate(2-ethoxyethyl-p-methoxycinnamate), diethanolamine-p-methoxycinnamate, digalloyltrioleate ethyl-4-bis(hydroxypropyl)aminobenzoate, 2-ethylhexyl salicylate, glyceryl aminobenzoate, homosalate(3,3,5-trimethylcyclohexyl salicylate, triethanolamine salicylate, 2-phenylbenzimidazole-5-sulfonic acid, sulisobenzone(2-hydroxy-4-methoxybenzophenone-5-sulfonic acid), Padimate A (amyl p-dimethylaminobenzoate), Padimate O (octyl dimethyl p-aminobenzoate), 4-t-butyl-4′-methoxydibenzoylmethane, the combination of 2-hydroxy-1,4-naphthoquinone with dihydroxyacetone and methyl anthranilate, in a cosmetically acceptable lotion, oil, cream, or emulsion.
Sunblocks, in contrast, generally contain an inorganic compound that reflects the sun's rays. The best known sunblock is zinc oxide, first formulated as a thick white cream famous on the noses of well-tanned lifeguards, but less obvious tinted zinc oxides and more user-friendly variations containing, instead, titanium dioxide, calamine, and fumed silica suspended in a cosmetically acceptable carrier have been described. (See, for example, U.S. Pat. No. 4,801,445 to Fukui, et al., and U.S. Pat. No. 4,822,600 to Wortzman; these and patents cited hereafter are incorporated herein in their entireties by reference.)
Since different organic compounds used as active ingredients in sunscreens absorb radiation of different wavelengths and with different efficiencies, sunscreens are unequal in their protection from both UVA and UVB rays. In addition, few sunscreens protect against visible or infrared (IR) radiation, which penetrate deeper than UV rays and can also cause damage. Therefore, the most effective protection against irradiation by the sun is provided by sunblocks, as these protect against radiation at all wavelengths. However, many sunblocks containing dispersed powdered inorganics such as zinc oxide and titanium dioxide are esthetically displeasing in that they spread poorly and have an unpleasant heavy and draggy feel when applied to the skin, and, since they are pigments, they can leave a white/blue residue, particularly at higher concentrations. Even when micromilled to ultra-micro particle sizes of 0.01μ to 0.15μ, titanium dioxide agglomerates into clumps of much higher particle sizes that are unacceptable for cosmetic use. Various dispersing agents and powder pretreatments have been suggested to improve the characteristics of formulations containing these sunblock inorganics such as coating the particles with vegetable oils, mineral oil, fatty acid esters or salts, phospholipids, silicone polymer films, branched chain organics, and the like. (See, for example, Fukui, et al., and Wortzman cited above, and U.S. Pat. No. 5,468,471 to Zecchino, et al., and other disclosures reviewed in these patents.) It would be beneficial to have alternative sunblocks exhibiting more desirable properties.